I want to knit bandeau top/cropped tube top with a stockinette front and a ribbed back to help keep it fitted, seamed together at the bust point. My bust is about 32 inches at the widest point (in the bra I want to wear with this top), and I suspect I'll need about 4 inches of negative ease to get the fit I want, as this yarn is really streatchy.

I want the stockinette part to be 12 inches wide in front, and the other 16 inches across the sides/back to be in ribbing. (I'm leaving out the 4 inches of negative ease)

My stockinette gague is 18 stitches and 26 rows = 4 in, about how many stitches do I need to cast on in 2x2 or 3x2 rib to get it to stretch to 16 inches?

I also welcome sugestions on which kind of rib would give me the kind of streatchy fit I want.

You'll have to knit a gauge in rib to find out. Cast on about 30 sts and do 2x2 ribbing for about 4", then switch to the 3x2 rib, though I think 2x2 is stretchier. Measure how many sts = 4" stretched and then multiply by 4.